Recently, in semiconductor manufacturing facilities, ozone gas generators have been used in large quantities for, e.g., cleaning of silicon wafers. Since even mixing of slight impurities or foreign substances significantly influences productivity in the semiconductor manufacturing facilities, a high-purity ozone gas generating capacity is required for the ozone gas generators used therein. For the foregoing reason, it is required that high-purity oxygen gas (e.g., the degree of purity of equal to or higher than 99.9%) is used as raw material gas and even mixing of slight impurities into generated ozone gas is completely prevented.
For example, there is a possibility that, if an electrode is exposed to a discharge gap, impurities are mixed into gas through a surface of the electrode. Thus, typically in a discharge cell of the ozone gas generator, e.g., an alumina dielectric is provided between the electrode and the discharge gap so that the electrode is not exposed to the discharge gap.
However, there is a problem that, if the mixing of impurities from the electrode is prevented and high-purity oxygen gas is used as the raw material gas to increase the degree of purity of ozone gas, high-concentration ozone gas cannot be stably generated (see, e.g., Patent Document 1). Facing such a problem, attempts have been made by, e.g., adding a slight amount of catalytic gas such as nitrogen gas to high-purity oxygen gas. However, since a nitrogen oxide which is undesirable in manufacturing of semiconductors is generated as a by-product, there is another problem that the foregoing ozone gas generator is not suitable as an ozone gas generator intended for a semiconductor field.
For the foregoing reason, a method for stably generating high-concentration ozone gas by using only high-purity oxygen gas which does not contain catalytic gas has been desired, and the applicant of the present invention has proposed various methods (see Patent Documents 1 and 2).
According to, e.g., Patent Document 1, it has been proposed that a predetermined amount of a titanium oxide is added to a dielectric. This allows stable generation of ozone gas even if high-purity oxygen gas which does not contain catalytic gas is used. In addition, according to Patent Document 2, it has been proposed that titanium (Ti), tungsten (W), antimony (Sb), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), vanadium (V), zinc (Zn), or an oxide thereof is effective and that powder of the foregoing component(s) is attached to a surface of a dielectric with a baking fixing agent made of glass.